The present invention relates to a new and improved construction of a constant pressure air storage installation with water supply for a gas turbine power plant.
Generally speaking, the constant pressure air storage installation for gas turbine power plants as contemplated by the invention is of the type wherein an air compressor can be driven independently of the gas turbine by an electric motor. Further, there is provided an underground cavern for storage of the compressed air and with connection lines extending between the cavern and the compressor and the gas turbine, respectively. Also, there is provided an equalization basin and a riser tube connecting the same with the cavern.
Constant pressure air storage installations for gas turbine power plants require, in relation to air storage installations of the same efficiency or output and working with variable air pressure, which can fluctuate during operation between certain boundaries, only approximately one-third of the volume of the latter. Hence, they are less complicated in their construction and the erection costs of a cavern for constant pressure air storages are appreciably less than in the case of caverns working with variable air pressure.
In order to maintain constant the air pressure of constant pressure storages there is used a water supply which compensates for the air volume which is consumed within the cavern. The water supply contains a water column which opens into a free basin located at ground level and whose static pressure gradient corresponds to the pressure which is to be maintained in the subterrainean cavern. During charging of the cavern, which with present day installations is located at a depth in the order of about 600 to 800 meters, corresponding to a static pressure of the water column of 60 to 80 bar, the water is forced up into the basin, and during discharge the water runs out of the basin into the cavern in order to ensure for the same pressure.
During the operation of air storage gas turbine installations it has been found that during the charging of the caverns the water column ascending in the water supply releases air which has dissolved within the water column. Hence, air bubbles are formed whose volume rapidly upwardly increases. These air bubbles cause a density reduction in the water column, and thus, a pressure drop in the cavern. In the extreme case the water column could be blownout by the compressed air cushion, and hence, the cavern could therefore become completely emptied.
In contrast to the normal speed of dissolution of air within static water, the complete saturation of the water with air occurs more rapidly within the cavern owing to the pronounced turbulence of the water during the charging and discharging operations, since now all of the water particles soon come into contact with the air. The quantity by weight of air which is taken-up by the water is proportional to the pressure, which, as stated, with the heretofore known installations, is in the order of between 60 and 80 bar. As to the thus dissolved quantity of air the following comparison is of interest:
At 1 bar air pressure and 10.degree. C. temperature 1 m.sup.3 water (=1,000 kg) contains 29.2 grams of air.
At 60 bar pressure and 10.degree. C. temperature 1 m.sup.3 water contains 1.7 kg air, in other words approximately 58 times the quantity by weight. At atmospheric pressure such 1.7 kg air corresponds to about 1.32 m.sup.3. A water-air mixture which has expanded from 60 bar pressure to atmospheric pressure therefore contains more air than water.
If water which has been saturated with air in this manner ascends upwardly out of the cavern, then by virtue of the descreasing hydrostatic pressure the air is released and forms increasingly greater bubbles. The average density of the water column therefore becomes increasingly smaller and the pressure within the cavern correspondingly drops. If there are not undertaken appropriate measures this can lead to blow-out of the compressed air cushion along with the water column.
A heretofore known measure for preventing this blowing-out phenomenon resides in extending the riser tube containing the water column downwardly in a U-shaped arc or configuration below the base or bottom of the cavern. The lowest point of the riser tube therefore must be located approximately 0.15 h below the relevant water level within the cavern, wherein h constitutes the effective pressure gradient, i.e., the difference between the geodatic height of the upper water level in the compensation or equalization basin and the cavern water level.
With the value of h=600 meters this would mean that the already 600 meters long riser tube must be extended downwardly at least another 90 metres, and specifically, twice this value for the down and up branches, something constituting an impermissibly great increase in the construction expenditure.